In the present study, we identified 27 (20.8%) and 103 (79.2%) cases as large duct and small duct type ICCs respectively, according to accepted classification criteria [10, 14], but the proportion of large duct type in ICCs was lower than previously reported [10, 15]. There are two possible explanations. Firstly, to ensure the accuracy of this research, we excluded many large duct type ICCs with characteristics similar to those of perihilar carcinoma. Secondly, ICC patients with cholelithiasis, often recognized as large duct type [15], were rarely admitted to our hospital.
Consistent with previous reports [11, 13, 15], we found that ICC patients with large duct type had worse prognosis than those with small duct type in univariate analysis. However, histopathological subtype was not an independent predictor of postoperative survival in multivariate analysis. Furthermore, the small duct type ICCs were closely associated with earlier tumor TNM stage. Accordingly, the bias of tumor stage from both subtypes may account for the inconsistency, and the histological subtype may not be an effective prognostic factor in patients with ICCs.
As reported in previous studies, IDH1/2 mutations were detected in 10.2–28.8% of ICCs [4, 6, 7, 29], and exclusively in small duct type of ICCs [10, 15]. In the present study, we found that 16.1% of patients with ICC harbored IDH1/2 mutation. Meanwhile, the incidence of IDH1/2 mutation in large duct and small duct type of ICCs was 3.7 and 19.4%, respectively. These results are consistent with previous reports. Consequently, the clinical features and prognostic value of IDH1/2 mutations should be investigated in small duct type, rather than in large duct type. However, previous studies have revealed inconsistent results regarding prognostic value of IDH mutation in ICCs. Yuchen Jiao, et al. [4] identified that ICC patients with IDH1/2 mutation had 3-year overall survival of 33% compared to 81% for patients with IDH1/2 wild-type (P = 0.0034), but Pu Wang, et al. [7] showed IDH1/2 mutation was significantly associated with improved prognosis in ICCs (4-years tumor recurrence rate was 45.3% vs. 71.5%; P = 0.046). At the same time, one study on advanced stage ICCs from Massachusetts General Hospital Cancer Center [29] found that there were no significant differences in DFS and OS between patients with IDH mutation and patients with IDH wild-type. In the present study, univariate and multivariate analyses showed patients with IDH1/2 mutation had significant prolonged DFS and a trend toward increased OS in small duct type of ICCs. Furthermore, IDH1/2 mutations were associated with higher histological differentiation, but not tumor stage. Therefore, these findings suggest that IDH1/2 mutation is a favorable predictor for small duct type ICCs. Due to worse prognosis of large duct type ICCs, cases with IDH1/2 mutation had significantly better DFS and OS than those with IDH1/2 wild-type in total cohort of ICCs. Our results may be account for the conflicting predictive values of IDH1/2 mutation in ICCs described above. In small duct type of ICCs, IDH1/2 mutations were correlated to a trend toward improved OS though the differences were not statistically significant. The inconsistence in DFS and OS maybe ascribed to the development of chemotherapy, immunotherapy, chemotherapy, local regional therapy and combination of different modalities, which obviously prolonged survival even after tumor recurrence. We further analyzed the treatments of ICC patients after recurrence and found that cases with small bile duct type were mostly to be treated with chemotherapy or palliative care. Coincidentally, a total of 9 patients underwent reoperation or radiofrequency ablation (RFA), all of which were classified as IDH wild-type. Therefore, this discrepancy between DFS and OS may be explained by the bias of different treatments after tumor recurrence in two groups. Moreover, the typical case described in methods who reached long-term (20 months) SD after treated with dasatinib also proved that ICC patient with IDH1/2 mutation had a better prognosis, even if the tumor is of advanced stage. In summary, IDH1/2 mutation may be a favorable predictor of small duct type ICCs, and have higher prognostic value for DFS than for OS.
Some investigators have found that IDH1/2 mutation was correlated to several clinical characteristics in ICCs, such as lower level of CA19–9, lower incidence of lymph node metastasis and smaller size of tumors [6, 29]. Interestingly, these clinical features were also observed in the small duct type of ICCs in our study. Since IDH1/2 mutations were typically detected in small duct type of ICCs, we considered these factors (CA19–9, lymph node metastasis and size of tumor) might be associated with small duct type, rather than IDH1/2 mutation. We firstly found that IDH1/2 mutation was strongly associated with decreased preoperative Fe and total bilirubin in small duct type of ICCs. Although the association of IDH1/2 mutation with iron and bilirubin metabolism has not been reported, previous studies have showed that in tricarboxylic acid cycle, iron homeostasis is regulated by haem and citric acid, both of which are closely related to the activity of IDH [30, 31]. Accordingly, the association between IDH1/2 mutation and iron metabolism may provide a new direction for the function study of IDH1/2 mutation in ICCs. Meanwhile, ICCs with IDH1/2 mutation, often categorized as small duct type, localized at the periphery of bile duct, rarely resulting in biliary obstruction in contralateral bile duct, which can explain the relationship between IDH1/2 mutation and lower level of serum total bilirubin.
It should be noted that IDH1/2 mutation is the most frequent mutant type of gliomas and can be detected by either DNA sequencing or IHC methods. Although DNA sequencing is regarded as gold standard, IHC seems to be more accurate, easier to perform and cheaper for detecting IDH1/2 mutation in glioma patients [32]. We firstly used MsMab-1 for detecting IDH1/2 mutation in ICCs by IHC method, and found that the results were basically consistent with those obtained by DNA sequencing. However, IDH1-R132C and IDH1-R132L, two common mutant types in ICCs, cannot be recognized by IHC so far, because their specific antibodies have not been commercially available yet. Therefore, multi-specific antibodies against hotspot mutations of IDH1/2 in ICCs need to be constructed and validated effectively in future.
Surgery remains the mainstream of potentially curative treatment for patients with resectable ICCs. Chemotherapy and radiotherapy are used as first-line therapy for advanced or relapsed ICC patients who are not candidate for surgery. Molecular targeted therapy and immunotherapy are developing very fast in the era of individualized and precision medicine [33]. Because of the low incidence of IDH1/2 mutations (3.7%), postoperative chemotherapy is recommended for ICC patients with large duct type without the need for gene sequencing. In contrast, due to the high incidence of IDH1/2 mutations (19.4%) in small bile duct type of ICCs, DNA sequencing should be recommend to detect IDH1/2 mutation and choose appropriate adjuvant therapies. If IDH1/2 mutation is detected, s small molecular medications that target IDH1/2 (dasatinib, ivosidenib or enasidenib) could be a promising treatment strategy [34,35,36]. With the development of specific antibodies against mutant IDH1/2, IHC will be an easy and cheap way to detect IDH1/2 mutation.
In addition, the clinical implications of other most frequent mutations (including BAP1, ARID1A and PBRM1) were investigated in each subtype of ICCs. Surprisingly, these inactive mutations were not significantly associated DFS and OS in small duct type of ICCs. Nonetheless, only BAP1 expression loss was correlated to prolonged DFS and OS in the large duct type of ICCs. Many studies [4, 25, 28] showed that these factors have no prognostic value in ICCs, but Sarcognato S et al. [8] found that the retained expression of BAP1 was correlated to poor prognosis, while another two studies [8, 9] got to conflicting conclusions about predictive value of ARID1A in ICCs. Accordingly, our study may explain the inconsistency in the prognostic value of BAP1: loss of BAP1 expression was correlated to improved survival in large duct type ICCs rather than in small duct type ICCs. When two subtypes were combined into one group, BAP1 can be considered as a favorable or irrelevant predictor in ICCs according to the ratio of large duct to small duct type ICCs.
There were limitations in this study. Firstly, the sample size is too small, especially for patients with large duct type ICC. This is why KRAS mutation, frequent in large duct type ICC, was not included in this study. Secondly, FGFR2-fusion, another frequent mutation in ICCs, was not investigated in this research.